Transistors, How do they work ?

The invention of transistors revolutionized human civilization like no other technology At the heart of a smartphone lies a processor And this processor holds about 2 billion transistors What do these incredibly tiny devices do? How do they work? Transistors can act like a switch with no moving parts They can amplify a weak signal, in fact amplification is the basic function of a transistor First let’s understand the basis of transistors,
we will come back to the application part later Transistors are made of semiconductors such as silicon Each silicon atom is bonded with four neighboring silicon atoms Silicon has four electrons in its valence shell Let’s replace the silicon atom with a four-handed smiley Each hand holds one electron Each one of these electrons goes for sharing with a neighboring silicon atom This is known as a covalent bond Currently the electrons are in their valence band If the pure silicon has to conduct electricity The electrons have to absorb some energy and become free electrons Thus the pure silicon will have a low electrical conductivity A technique called doping is used to improve the conductivity of semiconductors For example say you inject phosphorus with five valence electrons Here one electron will be free to move in the system This is known as N-type doping On the other hand if you inject boron with three valence electrons There will be a vacant position for an electron this vacant position is known as a hole and a neighboring electron can fill this hole at any time This electron movement is visualized as holes moving in opposite direction We call this P-type doping If you dope a silicon wafer in the following manner a transistor is born But if you really want to understand how a transistor works, we have to get a clear idea of what happens at the electron level of a more basic component, a diode A diode is formed when you dope one part of silicon as a P-type and the other part as an N-type Something very interesting happens at the boundary of the N and P joint The abundant electrons on the inside will have a natural tendency to migrate to the holes that are available on the P side This will make the P side boundary slightly negatively charged and the N side slightly positively charged The resulting electric field will oppose any further natural migration of the electrons Ff you apply an external power source across the diode as shown, the power source will attract the electrons and holes Electricity flow is impossible in this case However, if you reverse the power connection the situation is quite different Assume that the power source has enough voltage to overcome the potential barrier You can immediately see that the electrons will be pushed away by the negative terminal When the electrons cross the potential barrier, they will be drained of energy and will easily occupy the holes in the P region But due to the attraction of the positive terminal, these electrons can now jump to the holes nearby in the P region and flow through the external circuit This is known as the forward biasing of a diode Just keep this simple principle of a diode in mind, you will understand the operation of a transistor very easily Now back to the transistor Note that the P layer is really thin and lightly doped You can easily see that a transistor is essentially two diodes sandwiched back to back So whichever way you connect the power source, one diode will always be reverse biased and block the electricity flow This means the transistor is in the off state Now let’s connect a second power supply as shown The power supply should have enough voltage to overcome the potential barrier So this is just a forward biased diode Thus a high number of electrons will be emitted from the N region Just like in a diode a few electrons will combine with the holes and jump across the neighboring holes and flow to the base But there are a lot more electrons that has crossed to the P side, what will these remaining electrons do? Think for a moment The remaining electrons will get attracted by the positive terminal of the first power source and will flow straight as shown Note that the P region is very narrow which ensures that no remaining electrons flow to the positive terminal of the second power source In short a small base current is amplified to a high collector current You can easily correlate the naming of the transistor terminal with the nature of electron flow Ff you can increase the base current the collector current will increase proportionally, this is a clear case of current amplification The kind of transistor we have discussed is called a bipolar junction transistor Let’s replace this representative transistor with a realistic one You can further improve the amplification by introducing one more transistor Yhe base of this transistor is connected with the emitter of the first transistor Ff you introduce a weak fluctuating signal at the input like what you would find in a microphone, you will get an amplified signal at the loudspeaker The other interesting thing you can note about this basic circuit, is that depending on the value of the applied voltage, the transistor can be either on or off Here the transistor acts as a switch This property of the transistor opens the doors to the world of digital electronics and digital memory Using two BJT’s you can build the basic dynamic memory element of computer: a flip-flop To know more about it please watch the video on the transistor as a switch Please support us at so that we can continue our free educational service Thank you!

100 thoughts on “Transistors, How do they work ?”

  1. Boy did they over complicate that. If I didn't have a engineering degree in the stuff, I'd think I'd be really confused.

  2. Why when we use the same transistor but we reverse collector instead of emitteur I can't have the same current?

  3. I think theres a detail you skipped over
    The np junction is reverse biased so there will be negative charges on the right side of the n semiconductor and positive charges on the left side if the p semic.
    I guess that when u provide a positive voltage on the base the electrons on the right side of the p type are attracted to it, hence the depletion layer is disturbed and electrons can flow from the pn junction to the np junction

  4. Too many jumps, first one for me was going straight to saying valence shell. if someone doesn't understand what a transistor is, there is a good chance they don't know what a valence shell is either, like me 🙂

  5. Except this man does not actually understand how it works. Note that the collector base junction here is REVERSE BIASED. So really there should be no conduction then. If he knew how it worked he would have explained what really happens. Electrons that make it to the base become minority carriers in the base. Minority carriers WILL ALWAYS cross a reverse biased junction, so they all cross into the collector. This video simply says they get cross the junction as if it was a forward biased junction. Very bad.

  6. They alter a compound material to make composition material to allow easier flow of electrons. As the composite is a semiconductor. The voltage needs to be high enough for it to work. Hence it can be used like an on/off switch. Compared to a normal on/off switch which brakes the circuit and reconnects the circuit when switched.

  7. Can someone explain when it talks about how transistor is used as a switch? doesn't it depend on whether the power adapter is on and off? I don't get it when it focuses on "transistor" when it talks about the switch functionality.

  8. But one question remains. Why do you have emitter and collector. And why you can't put a transistor in the wrong way? If I turn NPN 180 °, I also have NPN. But that woun't work. Why?
    Can you please explain that in another part?
    Because your explanation was awesome. Like

  9. I have an electrical engineering bachelor's degree and I still can't grasp this video. However, this video is much better than how I was taught at the uni.

  10. The portion of the cartoon which uses a five-handed smiley to represent a neutral P atom is incorrect and misleading. It implies that phosphorous has five valence orbitals, and that each of those orbitals contains one electron. In fact,, phosphorous has four valence orbitals,* one of which contains two electrons. Furthermore, an atom which has five half-filled atomic orbitals has no special reason to donate an electron.

    Every student who has passed Introduction to Chemistry, Part 1 will know this. They will also know what VSEPR stands for and what the implications of that are for the correct mechanism for the operation of a transistor. Everyone except the authors of this video, it would seem.

    * Unless, of course the valence shell has been expanded, which is not the case here.

    Showing a hypodermic needle to represent "doping" is also rather disingenious.

  11. But why connected to the emitter, and also what is the difference between emitter and the other side? Is it that one gets power and the other uses it?

  12. Több, mint negyven éve próbálom megérteni a tranzisztor működését, és bár ezzel a filmmel már majdnem sikerült, de mégsem. Vagy a feliratot olvasom, de akkor nem látom az animációt, vagy az animációt nézem, de akkor nem látom a feliratot. Szóval most sem jött össze. 🙁

  13. It is nice to call emitter the p-part that in reality(logically) COLLECTS the electrons, if the genius who gave that name wanted to call it pretending that emitter part “emits” electrons into the base and then they get “collected” by the collector sadly he’s idea turned out to be very unintuitive.

  14. Wait, lemme get this straight. In order to understand transistors, you have to understand how a diode works first? A diode acts like a one way check valve in a plumbing system, right? It only allows current to flow in one direction. Well, at least I know how a diode works, lol.

  15. Other than the smileys (shaking in fear), this video helped me understand the transistor better than any other video I've seen yet

  16. Hmm but why did we need the transistor? Didn't the diode itself function as a switch? Couldn't we use that for logic gates?

  17. A bit confusing but still good I just need to watch again. I think an even simpler, and quick description of the general function of a transistor would make it easier to follow for beginners

  18. THANKS a lot😊
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    👆 👆
    👇 👇
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